Add Figma plugins section

[elioqoshi]

It will be good to add a section for recommended Figma plugins to get started, especially for UXers and Visual Designers.

Critical

Figmiro: https://www.figma.com/community/plugin/814814551050457760/Figmiro
Zeplin: https://www.figma.com/community/plugin/745330164019088593/Zeplin

Content

Google Sheets Sync: https://www.figma.com/community/plugin/735770583268406934/Google-Sheets-Sync
SPELLL: https://www.figma.com/community/plugin/754026612866636376/SPELLL---Spell-Checking-for-Figma-%26-FigJam
Content Buddy: https://www.figma.com/community/plugin/731260490045684148/Content-Buddy

Productivity

Super Tidy: https://www.figma.com/community/plugin/731260060173130163/Super-Tidy

[nottrobin]

We sort of have a separation between "guides" and "practices", where practices is supposed to be official policies and standards for team members to follow, rather than recommended or suggested tools and tips.

For now, I think it would be best to copy this list wholesale into a topic in https://discourse.ubuntu.com/c/design/guides/35.

However, I think (cc. @clagom) practices might be changing and moving to discourse or something before too long. We'll have to revisit how we define its purpose at that point I suspect.

[clagom]

Thanks @nottrobin, yes I think we need to add Figma in this form https://canonical-web-and-design.github.io/practices/workflow/UX-and-design-proposals.html @elioqoshi, to start with.
CC @amylily1011 as we talked about updating them.
Moving to Discourse would also definitely help, maybe something we can discuss during the planning week.